Plant Disease & Disorders

Plant disease is a process by which living and nonliving entities interfere, over a period of time, with a plant’s functions. This interference may cause changes in the plant’s appearance and/or it may bring about a lower yield than a healthy plant of the same variety. Plant disease can be indicated by a change in the plant’s appearance relative to a healthy plant of the same age and variety. Therefore, one must know what the plant normally looks like before determining whether it does or does not have a disease.

Disease causes changes in plant structure and function in the ways listed below.

1. Reduced photosynthesis.  The green plant manufactures its own food using sunlight, water, and carbon dioxide.  If a disease reduces the amount of light reaching the leaf, or if part of the leaf is killed, or if the leaf falls off prematurely, photosynthesis is inhibited.  Leaf spots, powdery mildew, anthracnoses, damage due to air pollution or pesticide toxicity, and twig blights reduce photosynthesis.
2. Root disease.  Roots, especially root tips, absorb nutrients and water while structurally supporting the plant. Browning, blackening, and limpness of roots are typical symptoms of root damage or root disease. Damage to roots results in yellowing, leaf scorching, slowing of growth, wilt, and dieback of aboveground portions of the plant. Fungi, bacteria, and nematodes, as well as excessive soil moisture, soil compaction, and excavation around the roots, cause root diseases.
3. Inhibited water and nutrient transport.  Water and nutrients absorbed by roots are transported to the trunk, branches, and leaves through vascular tissue. Water moves up the plant even to the tips of the leaves through the xylem. The food produced by the leaves moves back to supply the root cells by a parallel path down the plant through the phloem. If water and nutrient transport is disrupted, leaf tips and margins burn, leaves wilt, and roots die.  Inhibition of nutrient and water transport is caused by stem rots, vascular wilts, cankers, galls, and mechanical girdling.
4. Destroyed food reserves.  Perennial plants must have a food reserve to overwinter and energy to resume growth in the spring. Nutrients are stored in roots and stems. Destruction of food reserves is caused by root rots and stem rots.
5. Root diversion.  Diversion of food from plant growth to pathogen growth occurs when galls, mildews, nematodes, rusts, and viruses develop on and in the plant.
6. Inhibited reproduction.  Plant reproduction is inhibited in flower blights, stem blights, and many abiotic diseases.
   

Types of Disease
Diseases are either biotic or abiotic. Biotic, or infectious, diseases are caused by living entities known as pathogens. Pathogens reproduce, spread from plant to plant, and grow: if one plant has a biotic, infectious disease, the likelihood increases that a neighboring plant will get the disease. Living entities that can cause plant diseases are: fungi, bacteria, viruses, nematodes, viroids, mycoplasmas, spiroplasmas, and vascular plants (e.g. mistletoe, dodder and Indian pipes).
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Abiotic, or noninfectious, diseases are caused by nonliving entities, that is, things that do not grow, reproduce, or spread from plant to plant. If one plant has an abiotic, noninfectious disease, that does not increase the likelihood that a neighboring plant will get the disease. Abiotic diseases are caused by extremes of light, moisture, nutrients, and temperature, and air and water pollution. They may also be caused by pesticide and plant growth regulator toxicity, and by disruption of a plant’s root environment by soil compaction, excavation, and other human activities. Diagnosing an abiotic disease is difficult, since similar symptoms can be caused by many different things.

Fungi
Fungi, the largest group of pathogens, are multicellular organisms with a threadlike structure. The threads are called hyphae; when many hyphae mass together, they form a mycelium. They enter plants through natural openings or wounds, or penetrate directly through intact tissues.

Fungi contain no chlorophyll and therefore do not produce their own food through photosynthesis. Since fungi cannot manufacture their own food, they obtain nutrients from:
• Living tissues or other organisms; such an organism is termed a parasite; a parasite that can obtain nutrients only from living plant tissues is called an obligate parasite (e.g. rusts, powdery mildews, and downy mil- dews).
• Dead organic matter; such an organism is termed a saprophyte.

Many fungi have both parasitic and saprophytic stages in their life history.

Fungi produce spores. These sexual or asexual reproductive structures are capable of surviving adverse conditions and dispersing themselves. Sexual reproduction involves the mating of two separate cells or individuals; in asexual reproduction, no mating occurs. The way the spores are formed and their size, color, and structure are all used in identifying fungi. Many fungi form spores in characteristic structures called fruiting bodies. Fungi cause plant diseases by producing toxins that kill plant cells; by growing within and plugging up a plant’s vascular system; by causing roots to rot; or by invading plant tissues.
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Bacteria
Bacteria are single-celled microorganisms with cell walls. Plant parasitic bacteria lack chlorophyll and obtain nutrients from living or nonliving sources. None of the plant parasitic bacteria are now known are obligate parasites.

Plant parasitic bacteria are rod-shaped and reproduce by fission, that is, one cell pinches off to give rise to two cells after the DNA (genetic material) has replicated.

Bacteria enter plants through natural openngs (stomates, hydathodes, lenticles, and nectaries) or through wounds made by insect feeding, mechanical injuries, and pruning or grafting.

Nematodes
Nematodes are nonsegmented roundworms. They have a digestive system, a reproductive system, and a rudimentary nervous system but lack lungs and a heart. They reproduce by laying eggs. All plant parasitic nematodes have a spearlike mouthpart called a sylet and are obligate parasites. Most are small, less than 0.5 mm in length.

Four different life styles are found among plant parasitic nematodes:
1. Migratory ectoparasites: move throughout their life and stay outside the plant while feeding.
2. Migratory endoparasites: enter the plant and move inside its tissues throughout their life while they feed.
3. Sedentary ectoparasites: stop moving and establish a permanent feeding site where the remain outside tissues.
4. Sedentary endoparasites: establish a perma- nent feeding site within plant tissues and stop moving once established.

To determine whether nematodes are the cause of a plant disease, roots and other tissue must be collected and examined to detect endoparasites, while soil must be collected and assayed to detect ectoparasites.

Viruses
Viruses are composed of a piece of genetic material (either RNA or DNA) surrounded by a protein coat. They are submicroscopic and replicate only inside living cells. Viruses take over plant cell metabolism and direct the cell to manufacture more virus components. Viruses gain entry to cells either through a wound or during insect feeding.

Some viruses, such as a cucumber mosaic, die quickly if outside a cell or if the cell dies, while other viruses, such as tobacco mosaic, retain their ability to infect for years after the infected plant part dies. Many different viruses can infect plants.
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COMMON BIOTIC DISEASES AND SYMPTOMS
A symptom is a characteristic of a plant indicating that it is diseased. Symptoms may only be recognizable if healthy plants are near diseased ones. If all trees appear the same in an area, it gives the impression that all is normal. However, all may be stunted by the presence of nematodes on the roots or by damaging levels of air pollution. Some symptoms are microscopic and are not visible unless highly magnified.

External symptoms are visible on the outside of a plant and are usually obvious. Internal symptoms are inside the plant. For these to be detected the plant must be cut open. Local symptoms are those affecting a small area of the plant. Systemic symptoms affect a large portion of the plant or the entire plant.
Signs are the physical evidence of a pathogen. Fungal spores or mycelium, bacterial ooze from cankers, and water pooled around roots are all signs.

Leaf Spots, Blights, Scorch, and Anthracnose
• Leaf spots may be large or small areas of damaged leaf tissue. Spots may be yellow at first and then turn tan, brown, or black. Their centers may drop out, leaving a shot- hole appearance.
• Leaf blight is rapid killing of the entire leaf.
• Leaf scorch or marginal leaf burn refer to dead areas between the veins or along the margin of the leaf.
• Anthacnose refers to the dead areas on leaves, usually along leaf margins and veins. Spots frequently have concentric line patterns or dark, pimplelike fruiting struc- tures in them.

Sooty Mold
Sooty mold is a black fungal growth that develops on surfaces of trees and shrubs during the summer months. The appearance of sooty mold indicates that insects are present. Excrement from aphids, scales, and certain other insects contains sugars and nutrient-rich materials. The excrement drips onto needles, leaves, and twigs. The sooty mold then grows on this rich nutrient source. If the insect activity is eliminated, sooty mold will not be a problem; control the insect rather than the fungus.

Blights
Blights, the rapid killing of leaves and branches, are usually caused by bacteria and fungi. Since cankers on stems and branches, as well as root rots, can also result in the rapid death of leaves higher on the plant, affected plants must be examined carefully to determine where the initial infection occurred.

Important blights include fireblight on plants in the Rosaceae family (hawthorn, apple pear); late blight of potato caused by the fungus Phytophthora; bacterial blight of geraniums; and Botrytis blight caused by the fungus Botrytis.
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Rusts
Rusts are diseases caused by several different species of fungi that have dry, powdery, reddish-orange spores. Rusts are all obligate parasites. There are leaf rusts, gall rusts, and canker rusts. Rust fungi requiring two different species of host plants that must both be present for the fungus to complete its life cycle are termed heteroecious. Important rusts commonly found in the northeastern United States include cedar-apple, cedar-quince, and cedar-hawthorn rust, corn rust, wheat stem rust, white pine blister rust, hollyhock rust, snapdragon rust, orange rust on raspberry, and bean rust.

Smuts
Smuts are caused by fungi that are distantly related to rust fungi. Over the centuries, smuts have caused major losses of grain and cereal crops including barley, corn, and wheat. A common smut is corn smut. It is characterized by large swelling of white tissue, mostly on ears but also on stalks, leaves, and tassels. The black spores released late in the season from the galled tissue survive in soil on corn debris. Growing resistant varieties is the best control for smut. Smutted tissue should be destroyed before spores are released.

Galls
Galls can be caused by many different things, including insect feeding, tissue from wound healing, fungi, and bacteria. One important gall-causing organism is the bacterium Agrobacterium tumefaciens, which carries a plasmid that is transferred from the bacterium to the plant where it directs the plant to produce an unusual number of cells. Breaking the gall off is not a cure – most plants cannot be cured of galls. Galled plants, if particularly unsightly, should be removed. Otherwise, normal care will allow the plant to survive for many years.

Vascular Wilts
Most pathogens that attack the vascular tissues of plants are spread by insects, or they pass from plant to plant via root grafts. These pathogens survive from year to year primarily in infected plants, however, some survive for long periods in plant debris in the soil or free in the soil. Once inside the plant, the organisms causing vascular wilts are found almost exclusively in vascular tissue. The rate at which symptoms develop after infection varies with host plant, pathogen, and environmental conditions. Vascular diseases may kill a tree in one year or may debilitate the tree but never kill it. Herbaceous plants usually die by the end of the growing season.

The first symptoms of vascular disease are yellowing, wilting, or defoliation of one or two branches. As the disease progresses, larger and more branches show symptoms. Discoloration of vascular tissue in the outer wood is often seen.

Some important vascular diseases include Fusarium wilt of tomato and banana; Verticillium wilt of eggplant, tomato, and a large number of deciduous trees and shrubs; Dutch elm disease; and bacterial wilt of cucurbits.

Cankers
Localized areas of dead bark and the underlying wood on twigs‘ longer branches, and trunks are called cankers. Cankers can be caused either by living organisms (including fungi and bacteria) or by abiotic factors such as excessive low or high temperatures, hail, etc.

Many fungi that cause tree or shrub cankers normally inhabit the plant’s surface, gain entrance through natural or human-made wounds, and only cause disease when the plant is under stress. Some fungi, however, aggressively attack trees and cause cankers.
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There are three general types of tree cankers:
1. Annual cankers are caused by fungi that is not normally able to cause disease unless the tree is under stress and low in vigor. Infection can occur during the host’s dormant season. During the growing season, host callus tissues close off the canker and pre- vent it from spreading further. Although an- nual cankers do not persist, continued stress makes it likely that more cankers will form. (Example: Fusarium canker)
2. Perennial cankers are seldom lethal to the tree but they weaken its structure and de- tract from its appearance. The fungus invades wounds and branch stubs. The host forms callus around the infection site during the growing season, but the fungus invades more tissue. As this interaction continues, multiple ridges of callus create a target-spot canker. (Examples: Nectria canker, Eutypella canker)
3. Diffuse cankers are elongated, with little or no callus growth. Because the fungus invades so rapidly, the tree tissue at the edge of the advancing fungus is killed quickly. Branches or whole trees are girdled, sometimes in a single season. These diffuse cankers often kill the tree. (Exam - ples: Chestnut blight, Botryosphaeria canker, Phytophthora dieback, cytospora canker)

Damping-off
Damping-off is the rotting of seeds in the soil and the destruction of newly emerged seedlings by fungi. The entire seed may decay before germination or the seedling may rot just below the soil line. Older plants are usually not killed, but they develop stem and root rots. The fungi most often causing this disease belong to the genus Pythium; Fusarium and Rhizoctonia species can also cause damping-off.

Conditions that favor damping-off include excessive soil moisture and excessive misting; low soil temperatures before germination (below 68ºF or 20ºC); high soil temperatures after emergence (above 77ºF or 25ºC); and overcrowded flats or seedbeds.

To manage damping-off, start seeds and cuttings in pasteurized soil or a soilless mix using sterile flats or pots. Do not contaminate soil or containers by placing them on floors or dirty benches, or by using dirty tools. Buy fungicide-treated seeds or treat the seeds with fungicide. Finally, supply bottom heat so the soil in the containers is 70-75ºF (22-24ºC).
Certain fungicides in some rooting mixes can inhibit root initiation in some cultivars. It is best to avoid applying fungicides to cuttings being rooted.

Root Rots
Root rot can be caused by several different species of fungi, including Pythium, Rhizoctonia, Fusarium, and Phytophthora. These are common in field soil, sand and dead roots of previous crops. Almost all plants are susceptible to root rots at some stage of development. It is important to remember that anything killing roots will cause similar symptoms above ground. A yellow leaf can be a secondary symptom of root rot.

Symptoms are as follows:
• The base of cuttings are brown or black, and become soft.
• Plants are stunted.
• Root tips are brown and dead, and slough off when plants are pulled from the soil.
• Plants wilt at midday and recover at night.
• Plants yellow and die.
• Brown tissue on the outer portion of the root pulls off easily, leaving a bare strand of vas- cular tissue exposed.
• Root cells contain many microscopic, thick- walled spores.
• Stems rot at the soil line and have a dry, shredded appearance.

Since root rot is difficult to control once it has begun, efforts should be directed toward preventing the disease before it begins:
• Pasteurize potting soil and sand with heat treatments or purchase a soilless potting mix since these are generally free of pests.
• Disinfect all work surfaces, tools, and equip- ment that will contact the potting mix.
• Encourage vigorous growth since older plants are more resistant.
• Apply a fungicide.
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Virus Diseases
Depending on which virus is involved, the disease may be spread from infected plants to healthy plants in a number of ways. Once inside a plant, vegetative propagation perpetuates virus diseases: cutting taken from an infected plant usually are also infected even if no symptoms are immediately exhibited by the cutting. The virus particles are found in all parts of the plant except the few cells at the tips of the growing points. It is these few cells that are removed and grown into a healthy plant free of virus by the process called tissue culturing. Some viruses are carried on or inside the seed. When this occurs, usually less than ten percent of the seeds from an infected plant carry the virus.

Many viruses are spread by insects, and there can be a complex relationship between a virus and the insect that transmits it. Viruses carried on an insect’s sucking mouthparts are said to be stylet-borne; most mosaics are stylet-borne. Viruses carried inside an insect and eventually in the insect’s saliva are said to be circulative. Some circulative viruses actually multiply inside an insect and shorten its life.

Symptoms vary with the virus involved, the species of plant infected, and environmental conditions. Certain environmental conditions bring out symptoms, while others mask or hide them. Symptoms associated with virus infections are:
• Reduced growth, resulting in stunting
• Mosaic pattern of light and dark green or yellow on leaves
• Distortion and malformation of leaves or growing points
• Yellow streaking of leaves (especially mono cots)
• Yellow spotting on leaves
• Ring spots or line patterns on leaves
• Cup-shaped leaves
• Uniform yellowing, bronzing, or redding of foliage
• Breaking of flower color
• Distinct yellowing only of veins
• Crinkling or curling of leaf margins

Some of the above symptoms can also be caused by high temperatures, mycoplasmas, insect feeding, growth regulators, herbicides, mineral deficiencies, and mineral excesses. Most plants are susceptible to several different viruses. Virus diseases cannot be diagnosed on the basis of symptoms alone.

To guard against virus diseases:
• Purchase virus-free plants.
• Maintain strict insect and mite control.
• Control weeds, since they may harbor viruses, mites, nematodes, and insects.
• Destroy virus-infected plants.
• Frequently disinfect tools used for pruning.
• Propagate plants via seed rather than vegetatively.
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COMMON ABIOTIC DISEASES
Abiotic diseases are very common where plants are grown outside of their normal habitat, outside of their normal range, or in habitats that are disrupted by human activity. Any nonliving factor that disrupts a plant’s basic functions may cause disease.

To be healthy, plants require the following conditions:
1. Plants must have a balance between moisture and air around roots. Root cells need oxygen to respire or they die. All the cells of the plant must have moisture.
2. Soil conditions must allow sufficient root growth for water and nutrient uptake.
3. Nutrients in the soil must be sufficient for plant needs but not be so excessive that they become toxic.
4. A balance of required nutrients is necessary since an excess of one nutrient can induce deficiency of another.
5. Light must be sufficient for the plant to carry out photosynthesis, but not so excessive that chlorophyll is destroyed.
6. Conditions at the site in winter must be such that the plant can adapt and survive while maintaining vigor.

If one or more of these requirements are not met, the plant can be expected to exhibit symptoms of disease, including:
• general slowing of growth
• decline in vigor
• yellowing of older leaves
• branch dieback
• wilt
• marginal leaf burn or needle tip death
• premature fall leaf coloration
• smaller-than-normal leaves
• heavy seed production

Diagnosing an abiotic disease is difficult because similar symptoms can be caused by many different things. Accurate diagnosis requires that the observer observe all symptoms and note when they first appear, obtain a history of growing conditions; examine the site carefully; and examine neighboring plants of the same species and different species for similar symptoms (best done by the person maintaining the plants.).

Excessive Moisture and Poor Soil Aeration
Excessive rainfall or irrigation can cause symptoms typical of excessive moisture. However, if the soil is poorly drained, even small amounts of moisture can cause such symptoms. Soils with high clay content are poorly drained even on a slope.
Symptoms of excessive moisture include yellowing of older leaves or needles, defoliation, wilting although soil is moist, and limp roots. Signs include algae or moss growing on the soil surface, soggy soil, and soil that is dark olive green to olive brown in color 6 to 9 inches below the surface.

Insufficient Moisture
Disease caused by insufficient moisture can result from not watering deeply or insufficient rain, weather that dries the plant faster than the root system can supply moisture, and when plants are not well rooted after transplanting. If the planting site is restricted in size (shallow to bedrock or paving all around the root system) or if the drainage pattern is changed, sufficient moisture may not be available to the plant. If the root system size is reduced due to excavation, root rot, nematode feeding, or soil compaction, the plant will not be able to take up sufficient moisture. Also, if the soil is frozen while air temperatures are mild, broadleaved and needled evergreens will continue to lose moisture yet be unable to replenish their supply.

Symptoms include yellowing, wilting, premature fall leaf coloration, and marginal leaf burn. Signs include soil that is very dry, dusty, hard, or cracked. The potting mix in containers may be pulling away from edge of pot or the mix may not be moist entirely.

Plants in very windy, exposed locations or in containers are very susceptible to problems caused by insufficient moisture.
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Excessive Salts or Overfertilization
While excessive salt is primarily a problem in containerized plants, similar symptoms are caused in the landscape by the presence of deicing salts. Symptoms include yellowing, wilting, defoliation, marginal leaf burn, leaf tip burn, and stunting. The signs are a crust of salt on the soil surface or a crust around the edge of the container, an excessive amount of slow-release fertilizer pellets on the soil surface, and damage to plants primarily on the side closest to a road or walk that is treated with deicing compounds.

Plants vary greatly in their tolerance to high concentrations of salts. Sensitive plants include American elm, basswood, red and white pine, hemlock, red and sugar maples, shagbark, hickory, roses, hollies cotoneaster, Podocarpus, Photinia, and viburnum. Moderately tolerant plants include Austrian and pitch pines, birch, red and white oak, black cherry, white ash, poplar, privet, juniper, pyracantha, boxwood, and Taxus. Good tolerance is exhibited by euonymous, Russian olive, thornless common honey locust, Japanese black pine, and black locust.

Heavy watering on two or three consecutive days can significantly reduce the salt level: six inches of water reduces salts by 50 percent; 12 inches reduces salts by 60 percent; and 24 inches reduces salts by 90 percent.

Nutrient Deficiencies and Imbalances
Plants require certain nutrients in varying amounts. Major elements (macronutrients) – those required in relatively large amounts – include nitrogen (N), phosphorus (P), potassium (K), calcium (Ca), magnesium (Mg), and sulfur (S). Minor elements (micronutrients) are required in lesser amounts, including iron (Fe), boron (B), manganese (Mn), copper (Cu), zinc (Zn), and others. Symptoms of deficiencies vary with host species; in trying to diagnose nutrient problems soil and tissue tests should also be run.

Nutrient deficiencies can be caused by: lack of nutrients; soil pH that makes the element unavailable to the plant; and an imbalance between two elements, disrupting intake. As an example of the last factor, excess calcium leads to magnesium deficiency; nitrogen deficiency can induce magnesium deficiency; and potassium deficiency can cause excessive uptake of molybdenum.

Symptoms of major element deficiencies are as follows:
• Nitrogen deficiency usually results in yellowing of older leaves, small leaves, and stems that are short and slender.
• Phosphorus deficiency causes purplish to reddish coloration in leaves and stems, or the leaves may appear unusually dark green. There is a general stunting of plants, distor- tion of leaves, and death of leaves, petioles, or fruits.
• Potassium deficiency causes older leaves to be mottled, spotted, or curled. Affected leaves may have scorched leaf tips and mar gins or the tip of the leaf may curl down- ward. Root systems are poorly developed.
• Magnesium deficiency causes yellowing between the veins of older leaves as well as causing spotting of leaves.
• Iron deficiency results in yellowing between veins of younger leaves while veins remain dark green.
• Manganese and iron deficiencies are often misdiagnosed. Excess manganese can in- duce iron deficiency. Leaves are smaller than normal, young leaves are yellowed between the veins while veins are green, and leaves fall prematurely.

Excessively High Temperatures or Light Conditions
High temperature damage can occur when a plant:
1. is not acclimated to its location because the plant was recently moved to that new loca tion ;
2. was moved from greenhouse or shade house to full sun; or
3. was recently exposed to full sun by the re moval of overstory trees.

In some cases, a building or other structure reflects heat and light onto the plant, or the plant is exposed to heat exhausts from air conditioners, clothes driers, or steam lines. To diagnose the problem, compare the date of a sudden change in environmental conditions to the date symptoms began. Note whether symptoms are more severe on one side of the plant than another. Symptoms include a light green coloration of leaves, bleaching of leaves or bark, mottling or uneven green coloration on leaves, death of leaf tissue or roots, and dead tissue at the soil line of seedlings. Protect plants while they are being acclimated to a new location. Use shading or frequently mist the plant with water during the heat of day. Position a lathe or cloth between the plant and the heat-reflecting structure until the plant grows large enough to shade a portion of that structure. Mulch the base of seedlings in hot locations.
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Excessively Low Temperatures
Damage depends on when the low temperature occurs. Continuous cold around hardy, acclimatized plants in winter causes little problem. However, widely fluctuating temperatures do cause problems. Symptoms include the blackening of young tissues, uniform browning of evergreen tips, marginal leaf burn of broadleaved evergreens, reduced flowering, and death of a plant that was healthy the previous autumn.

Allow plants to harden by not applying excessive nitrogen in the fall.

Use plant hardiness zone maps to select plants for particular locations. Even within a given hardiness zone, if a site is particularly harsh, consider using only plants adapted to a lower number hardiness zone.

Water plants in late summer and fall to prevent them from entering winter under drought stress. Drought predisposes plants to winter injury and cankers. Pack potted plants close together and cover them with a translucent plastic sheet that does not touch plants. Mulch or mound soil around pots, and ball and burlap plants to insulate roots. Wrap burlap or build a lathe structure around plants in exposed locations.

Air Pollution
Symptoms of air pollution damage can develop from a single acute, highly concentrated dose or from persistent, low-concentration, chronic doses. The symptoms that develop depend on the plant affected, the stage of plant growth affected, the time of year, and the identity of the pollutant.
• Ozone (O3) occurs naturally in air but can also be produced by auto emissions, light- ning, or electrical discharges. Plants are most susceptible to damage in the of ozone injury include death of conifer needle tips in acute exposure and yellowing in chronic exposure. Flecks of white to tan spots on the upper surface of the leaves of broadleaved plants are common. Ozone- sensitive species include Virginia pine, jack pine, Austrian pine, Scotch pine, white pine, hemlock, larch, green ash, white ash, honey locust, mountain ash, pin oak, scarlet oak, white oak, poplar, redbud, sweetgum, and sycamore. Ozone-resistant species include arborvitae, Douglas-fir, balsam fir, white fir, red pine, spruce, birch, dogwood, linden, maple, English oak, and red oak.
• Sulfur dioxide (SO2) is produced by fossil fuel combustion. A reddish-brown needle- tip burn develops on conifers exposed to high concentrations. This is followed by premature defoliation of old needles. Chronic levels will cause a yellowing of tissue. Yellow or bleached tissue appears between the veins of broadleaved trees. Sensitive species include apple, birch, catalpa, larch, white pine, and poplar. Arbor vitae, juniper, and maple tend to be resistant.

Fluorides
Manufacturing of steel, bricks, tiles, and cement releases fluorides. Broadleaved plants take up fluorides and transport them to leaf tips and margins. As the tissue fades and dies, a reddish-brown band separates the injured from the healthy tissue. Pine needle tips turn yellow, then tan, and finally reddish-brown.

Ethylene
Leaking gas pipes can be a source of ethylene. Growth reduction, bud death, and abnormalities in growth are caused by ethylene. Twisting of young stems and flower stalks, defoliation of plants (in an enclosed area), death of roots, and dieback all can occur. Ethylene-sensitive species include Douglas-fir, white pine Scotch pine, and mugho pine. Resistant species include birch, cherry, dogwood, American and Chinese elm, juniper, linden, poplar, sweetgum, sycamore, and willow.
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ENVIRONMENTAL STRESS
Many organisms, especially facultative parasites (those which do not have to be parasites in order to live), are not extremely aggressive and attack plants only under certain conditions. Diseases caused by these organisms often only develop when a plant is not vigorous, or when conditions are not optimum for the plant to react to attack. In the landscape, these environmental stress-related diseases are the most common of those encountered.

The living organisms most often involved are fungi. Some environmental stress is required for them to be able to cause disease. The stress alone does not kill the plant but weakens it, predisposing it to disease. For example, canker-causing fungi may invade tissue during a plant’s dormant season, when the plant’s reaction to invasion is slow and the plant is low in vigor. Also, most canker-causing fungi are not able to cause disease unless the plant is under an environmental stress such as drought. Root pruning that occurs during transplanting causes a droughtlike stress and predisposes woody plants to cankers.

Attack by insects can stress plants and predispose them to fungal attack. For example, repeated defoliation of oaks by gypsy moth larvae weakens trees and predisposes them to Armillaria root rot.

A decline is the gradual reduction in growth, dieback, and final death of a population of plants. Declines
• occur well within a plant’s normal geographical range and not just at its fringes
• are occurring when five percent or more of the population is lost per year over several years
• occur in 10- to 25-year cycles.

One theory proposes that three factors are involved in declines:
1. Long-term factors that change very slowly impose a stress; these include site character- istics, soil type, soil pH, climate, and host genetics.
2. Short-term factors that change greatly from year to year add to the stress; these include drought, flooding, and injuries.
3. Contributing factors actually kill the plants; these include diseases and insects, especial- ly weak parasites, and secondary fungi. Contributing factors are readily found and receive the greatest emphasis, but without the long-and short-term stresses no decline would occur.

Symptoms of decline are:
• reduced growth: shoots are shorter, especially on evergreens, and annual rings are smaller (reduced radial growth)
• roots and mycorrhizae degenerate
• food reserves are depleted compared to those of nondeclining trees
• leaves color prematurely in fall
• winter twig diebacks develop and are invaded by weak parasites
• portions of the canopy die
• water sprouts develop on main stem and branches of hardwoods
• root rot and decay fungi attack
• secondary insects attack, including borers and bark beetles

To manage a declining plant, alleviate the short-term stress factors if possible. Fertilize, irrigate, drain, or mulch as appropriate. Protect weakened plants from contributing factors by controlling leaf diseases, especially those causing defoliation and twig death, and by controlling defoliating and boring insects.

MANAGING PLANT DISEASE
Successful plant disease management entails a three-pronged approach:
1. Monitoring. The objectives of monitoring, or examining, plants is to avoid introducing a pathogen into an area, to detect and erad- icate any disease found, and to detect diseases that cannot be excluded or eradicated early so that other appropriate actions can be taken.
2. Prevention. The objectives of preventive techniques are to prevent disease from developing in the first place. Such techniques include purchasing and planting pathogen-free material, treating soil before planting to eliminate disease-causing organ- isms, and, if necessary, applying chemicals before diseases normally appear.
3. Management. When a disease is detected, take actions to manage the disease by reduc- ing either its severity or its spread.
Many different control measures can be used in managing diseases. For most diseases, more than one type is required. Integrating several types of controls usually is more successful than relying on only one method.

Regulatory Methods
Regulatory methods of disease control attempt to exclude disease-causing organisms, highly susceptible host plants, or alternate host plants from a growing area. Inspection and quarantine of plant materials are tools for enforcing rules and regulations that prevent the introduction of a disease-causing organism or a disease-carrying plant into an area. A list of plant materials that cannot be brought into the United States is available from the Customs Service.

Inspections can be done at the point of origin before the material is packed and shipped or at the port of entry or receiving site upon arrival. Trained inspectors are required to watch for known pests on particular plants, for indications of problems due to some unknown cause, or for unwanted plants.

Quarantine is the holding of plants, potting materials, or packing materials for a period of time to allow a disease to develop or a disease-causing organism to grow so that it is detectable.
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Cultural Methods
Some of the more common cultural methods are closed season and dry fallow, crop rotation, sanitation, manipulating the environment, and improved plant vigor.
Closed season and dry fallow
A “closed season” is a period in the greenhouse or field when no host plants are present. Having a closed season is especially effective when obligate parasites are of concern. Dry fallow involves soil tillage to prevent weed growth and to keep the upper layers of the soil dry. Nematodes like the lesion nematode and some bacteria cannot survive such conditions.
Closed seasons and dry fallow are not effective against soilborne pathogens such as Pythium, Phytophthora, Verticillium, or Fusarium. These organisms produce spores that are very resistant to adverse environmental conditions. Resistant spores allow the organisms to remain in a living but dormant state for long periods of time. Closed seasons and dry fallow are also ineffective against foliar pathogens, such as powdery mildew or Botrytis. These fungi produce wind-disseminated spores that readily recontaminate the area.
Crop rotation
Crop rotation is a modification of closed season. Plants may be present, but they are not host plants. Thus, the land is still in use, but the crop being grown is not threatened by a certain pathogen. An example of a poor rotation is tomato followed by eggplant followed by potato since all are susceptible to similar pathogens. A better rotation is tomato followed by corn, beans, or lettuce.
When selecting plant replacements for landscape sites, the gardener should consider previous rotations, because the organisms that killed the original plant may still be present in the soil or on surrounding host plants.
Sanitation
Sanitation involves reducing the amount of inoculum present, destroying sites where pathogens overwinter, removing noncrop plants that harbor pathogens, and disinfesting equipment. Raking fallen leaves, rouging (removing infected plants entirely), burning debris, pruning infected twigs and branches, covering fallen leaves with mulch, and cleaning soil and plant sap from tools are all forms of sanitation. Sanitation is useful for managing cankers, bacterial diseases, vascular wilt diseases, and some foliar disease. When tools are used to handle infected plants or to prune out infected tissues, they should be viewed as being contaminated with the pathogen. As such, they may spread the pathogen to the next plant they contact. Similarly, when tools and equipment are used to handle pasteurized soil, they should first be cleaned and disinfested so that weed seeds and pathogens are not brought back into the soil.
Manipulating the environment
If environmental conditions can be manipulated to favor plant vigor and to be unfavorable for disease development, certain diseases can be completely controlled and, for many diseases, the effectiveness of additional management techniques enhanced.
Controlling humidity to reduce the duration of leaf wetness greatly influences foliar disease development. Weed control and plant spacing influence air circulation in and around the planting site. The irrigation method used influences the duration of leaf wetness. Drip or trickle irrigation keeps plant surfaces dry and therefore unfavorable for most disease-causing bacteria and fungi. Sprinkle irrigation wets the foliage, providing moisture for fungal and bacterial activity, and also splashes these pathogens from place to place.
Controlling soil moisture strongly influences the development of Pythium and Phytophthora root rots; both fungi require wet soil to develop. By draining excessive moisture away from plants, soil conditions become less favorable for these fungi.
Improving plant vigor
Watering, fertilizing, preventing soil compaction, preventing damage from excavation or pedestrian traffic, and other practices that promote plant vigor strengthen the plant’s ability to resist disease. Maintaining plant vigor is extremely important in declines, diebacks, and canker diseases. For example, steps should be taken to prevent winter injury, maintain root and shoot balance when transplanting, mulch to retain moisture at dry sites, drain excessively wet sites, and reduce tillage to avoid root injury.

Resistant Varieties
One of the most effective ways to prevent disease is to plant varieties that are resistant to the pathogen. Plants may be susceptible, tolerant, or resistant to various pathogens. The term susceptible indicates that the plant readily becomes diseased if the factors of environment, time, and pathogen are favorable. The term tolerant implies that the plant may become diseased but is not severely affected.
Resistant plants do not readily become diseased unless environmental factors are extremely favorable to the pathogen. Plants that never get a particular disease are said not to be hosts of that particular disease. Since plants do not have an immune system, the term nonhost is more appropriate than immunity in describing a plant that is not at all susceptible to a particular pathogen pathogen.
Plant resistant cultivars when minimum maintenance is desired; inoculum is always present; environmental conditions usually favor disease at some time; chemical controls are not available, are not likely to be used in a timely fashion, or are very difficult to apply; and plants become severely disfigured by disease.

Biological Controls
In biological control, one organism is used to attack or at least inhibit the activity of another organism. In hyperparasitism, one organism is introduced to parasitize or kill another organism. Examples are:
• Bacteriophages are viruses that infect bacteria.
• Bactericins are chemicals produced by one bacterium that kill another bacterium.
• Mycoparasitism occurs when a fungus attacks another fungus or a nematode.
• Cross-protection occurs when a plant infect- ed with one strain of fungus or virus is made less susceptible to another fungus or virus.

In biological control, a certain level of disease must be tolerated, since (a) the control agent must have a nutrient source; (b) the control agent may have to be frequently reintroduced if it is so efficient that it eliminates its food source and dies out itself; and (c) most control agents only work well under very specific environmental conditions. These disadvantages must be weighed against the advantage of reducing or eliminating the use of some chemical pesticides.
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Physical Controls
Heat treatments can be used to reduce the numbers of pathogens in soil or on plants. Seeds, bulbs, and rootstocks can be heated to a point that kills the pathogen but not the plant tissue.
Refrigeration is used to chill plants and retard the development of pathogens. This technique is most often used when plants or plant parts are being held in storage.

Chemical Controls
The objective of chemical disease control is to eliminate or inhibit pathogen germination, growth, invasions, colonization, reproduction, or spread.
Modes of action
Chemicals may kill the pathogen directly, may only inhibit pathogen activity to allow plants to grow to a stage of lessened susceptibility, or may induce a resistant reaction in an otherwise susceptible plant and not be toxic or inhibitory to the organism directly.
Most chemicals are protectants and must be applied before or very soon after disease begins. To be effective, chemicals must come in direct contact with pathogen. This requires that contacts be applied uniformly on the plant surface and frequently so that new tissue is protected. Contacts are generally applied as sprays, dusts, seed treatments, fogs, or mists to aboveground portions of plants, or as granulars or dusts incorporated into soil.
Systemics are chemicals that enter the plant and are distributed throughout the plant. Uniform coverage is not as important as with contacts, although it is helpful. Reapplication is usually less frequent since the chemical is inside the plant; is not weathered away by sun, wind, and rain; and is redistributed to newly forming plant parts. Systemics must have a high level of compatibility with the host because they are absorbed and translocated within the plant.
When to apply
The most important aspect of using chemicals effectively is to apply the appropriate material at the proper time. Therefore, the cause of the disease must be known so that the proper chemical can be selected. Also, the biology of the pathogen must be known so that the chemical can be applied when the pathogen is in a stage of development sensitive to the chemical. No single chemical is effective against all pathogens, and most pathogens have certain stages of development that are extremely resistant to adverse environmental conditions, including the presence of inhibitory chemicals. If the wrong chemical is applied, or if the appropriate chemical is applied at the wrong time, chemical control will fail.
Phytotoxicity
A common result of applying chemicals for disease and insect control is damage to the treated plant, an effect termed phytotoxicity. This may occur when chemicals are used to protect plants from pests, to fertilize plants, or to regulate plant growth. Phytotoxicity can occur when:
• a material is properly applied directly to the plant during adverse environmental condi tions
• a material is applied improperly
• spray, dust, or vapor drifts from the target plant to a sensitive plant
• runoff carries a chemical to a sensitive plant
• persistent residues accumulate in the soil or on the plant.
Symptoms of phytotoxicity include:
• poor seed germination, especially if a soil drench was used
• death of seedlings
• death of rapidly growing succulent tissue
• stunting or delayed plant development
• misshapened or distorted plants, fruits, or leaves
• russeting or bronzing of leaves or fruit
• dead spots or flecks on leaves
• dead leaf tips or margins
• dead areas between the veins of leaves.

Usually, the injured tissue is sharply defined, with little or no gradation from dead areas to healthy areas. That is, the dead spots are of uniform color and go entirely through the leaf.
You can avoid phytotoxicity by:
• being certain that the crop being treated is listed on the chemical container label
• measuring and preparing chemicals for use carefully
• being certain to use the proper concentration
• using the recommended application tech- nique
• applying chemicals after the heat of the day and when conditions will favor drying of leaf surfaces
• using low rates of chemicals on seedlings
• applying the chemical in a manner to prevent it from contacting adjacent or down- wind plants that may be sensitive
• not applying the chemical more frequently than the label recommends.

From “Plant Disease and Disorders,” Rhode Island Sustanable Gardening Manual, 2006

Plant Health Problems
Diagnosing Poor Plant Health
Identifying Problems of Garden Flowers
20 Questions for Diagnosis

 

Usefull Websites

Plant Disease Control, An Online Guide (Oregon State University Extension)
Plant Disease Diagnostic Clinic (Cornell University)
Plant Disease Diagnostics (University of Minnesota Extension Service)
Plant Disease Fact Sheet Index: Home, Yard and Garden (Ohio State University Extension)
Plant Diseases (Virginia Tech Cooperative Extension)

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